In projection mask aligners of the type described in U.S. Pat. No. 4,068,947 it is required to illuminate an arcuate field in order to expose a wafer to mask patterns during a scan operation. This is necessary since, as more fully set forth in the above-mentioned patent, the projection optics described therein are fully corrected only within an annular ring concentric with the optical axis. Thus, a light source must illuminate an area of the object plane of the imaging system where optimum imaging properties exist, i.e., the area of the fully corrected annular ring. In actual practice, only an arcuate section of the annular ring is used for imaging purposes with the rest of the ring and remaining field blocked out. Due to the arcuate shape of the area of good imagery, efficiency and uniformity considerations have in the past dictated that the light source used in exposure be arcuate in shape.
In general, the arcuate shaped exposure light source has been obtained by using bent gas discharge tubes as lamps which have a high degree of brightness and which provide radiation in the ultraviolet region of the spectrum required for exposure of the photoresist on a wafer.
Difficulty of manufacture causing high initial costs and short operation life make such lamps a high expense item for device manufacturers. Ideally, use of an ordinary short arc lamp would substantially reduce costs. However, in order to utilize a short arc lamp, means are required to shape the light therefrom in such a way that an arcuate field is illuminated uniformly.
It is known that a band-like section of a spherical mirror or elliptical mirror will convert light from a point light source, such as a short arc lamp, into an arcuate image. For example, see U.S. Pat. No. 3,529,48 issued Sept. 15, 1970 to N. M. Stefano et al "Collector And Method For Producing A Nearly Uniform Distribution Of Flux Density On A Target Plane Perpendicular To The Optical Axis" for use of a band-like portion of a mirror whose surface is a figure of revolution about an axis to form an arcuate image in a plane perpendicular to the axis. U.S. Pat. No. 4,292,663, issued to Martino et al Sept. 29, 1981 for "High Intensity Illumination Light Table" discloses a toroidal elliptical mirror arrangement for forming an arcuate image. U.S. Pat. No. 4,294,538 issued to Ban on Oct. 13, 1981 for "Image Forming Optical System" also discloses the formation of an arcuate image from a point source utilizing a band-like segment of a spherical mirror.
None of these publications treat the essential problem solved by Applicant, i.e., preservation of uniformity of illumination of the short arc lamp in the arcuate image and efficient use of the lamp's light by collecting energy over most of the 360.degree. emission field while at the same time providing uniform imagery by reducing or eliminating the variation of numerical aperture with the azimuth of the imagery.